The present invention relates to a process of reversal development of an electrostatic latent image by adhering powdery developer on the portions of the latent image having the lesser electrostatic charge, and more particularly to a process of reversal development using a single component, i.e., inductively chargeable, magnetic toner and an apparatus for carrying out the process.
In an electrostatic developing system, an electrostatic latent image is ordinarily pictured on a support member of a photoconductor. The latent image is produced by the steps of charging positively or negatively the photoconductor, such as non-crystalline selenium or the like, in a uniform manner, exposing a light image to be reproduced on the uniformly charged photoconductor, thus reducing the charge on the exposed portion and maintaining the charge on the non-exposed portion substantially as such. Alternatively, it has also been conventional to produce an electrostatic image by loading a charge on a highly resistant sheet, such as an electrostatic recorder sheet.
In the development of the latent image, the term "normal development" refers to a process wherein colored powder, which is called a toner, adheres to the higher potential portions corresponding to the unexposed portions in the electrostatic latent image produced in such a manner, the difference in potential being an absolute value. On the other hand, the term "reversal development" refers to a process in which the toner adheres to the lesser potential portions, the lesser charged portions corresponding to the exposed portions.
Recently there has been an increased demand for reversal development to be used, for example, in microfilm printers, facsimiles and laser beam printers. In a process of a development of prints by microfilm printers, the latent image is developed with toner which is formed on a photoconductive paper with exposure to a light pattern through a negative film. As the negative film has transparent characters on opaque background, the latent image formed on the paper comprises discharged areas corresponding to the characters and charged areas corresponding to the background. Upon the development of the latent image, it is necessary to adhere powdery developer to the discharged areas in order to obtain a positive visible image. And also, in some cases of development of facsimiles and laser beam printers, it is well-known that powdery developer should adhere on discharged areas on a member supporting the electrostatic latent image.
Such a reversal development in the prior art was shown in U.S. Pat. No. 3,888,666 to Matsumoto et al. The patent disclosed that a photoconductor can be used as an electrode facing the photoconductor having a latent image. The photoconductor, used as an electrode, is the same material as that of the photoconductor for the latent image and has the same charge as that originally charged on the photoconductor for the latent image. The latent image is negatively developed by supplying a developer powder dispersed in an insulating liquid which is charged beforehand in the same polarity as the latent image.
On the other hand, there has been recently developed a process for normal developing with a toner containing magnetic particles therein in place of conventional developer, such as liquid developers and two component developers which comprise colored powder and carrier. The newly developed toner, so-called single component magnetic toner, which is inductively chargeable, has proved popular because it does not need mixing of conventional colored powder and carrier particles. It results in reducing the prior complicated structures of the developing apparatus.
Representatively, such a process for normal development is disclosed in U.S. Pat. Nos. 3,909,258 and 3,816,840 both to Arthur R. Kotz. In this process, toner particles are used exclusively for the development without the use of carrier particles. The toner particles, which contain ferromagnetic particles, are conveyed to the developing position, during absorption onto an electroconductive shell incorporating a permanent magnet, to form a magnetic brush at least at the developing position. Electrically conductive circuits are formed between the toner particles and the electroconductive shell and the electroconductive shell is grounded to the frame of the duplicator so that an electric charge having an opposite polarity is induced on the toner brush by the action of the electric field caused by the electrostatic charge on the support member for the electrostatic latent image. In other words, as the toner on the magnetic brush is grounded, a positive charge is induced on the toner where the latent image on the support member bears a negative charge. However, it is very difficult to obtain a reversal development in which the inductively chargeable toner adheres to the lesser charged areas of the electrostatic image. Since the so-called single component toner is inductively charged by an external electrical field, the toner tends to adhere to the more highly charged areas, rather than to the lesser charged areas of the image.